13
Aggregate Grades of Evidence by Question Type
Grade Treatment Diagnosis Prognosis
A
Systematic review
a
of
randomized trials
Systematic review
a
of
cross-sectional studies
with consistently
applied reference
standard and blinding
Systematic review
a
of inception cohort
studies
b
B Randomized trials, or
observational studies
with dramatic effects
or highly consistent
evidence
Cross-sectional studies
with consistently
applied reference
standard and blinding
Inception cohort
studies
b
C Non-randomized or
historically controlled
studies, including
case-control and
observational studies
Non-consecutive
studies, case-control
studies, or studies with
poor, non-independent,
or inconsistently applied
reference standards
Cohort study, control
arm of a randomized
trial, case series, or
case-control studies;
poor quality prognostic
cohort study
D Case reports, mechanism-based reasoning, or reasoning from first principles
X Exceptional situations where validating studies cannot be performed and there
is a clear preponderance of benefit over harm
Level of Confidence in Evidence
H High M Medium
a
A systematic review may be downgraded to level B because of study limitations, heterogeneity, or
imprecision
b
A group of individuals identified for subsequent study at an early, uniform point in the course of
the specified health condition, or before the condition develops
